Decorative coating materials, or what are known as decorative papers or decorative films, are preferably used for surface coating in furniture production and in interior design, in particular laminate flooring. Decorative papers/decorative films are understood to mean printed or unprinted papers impregnated with synthetic resin or impregnated with synthetic resin and also surface-treated. Decorative papers/decorative films are glued or laminated to a carrier board.
Depending on the type of impregnation process, a distinction is made between decorative papers/decorative films with fully impregnated paper core and so called pre-impregnates, in which the paper is only partially impregnated online or offline in the paper machine. None of the previously known pre-impregnates containing formaldehyde-containing thermoplastic resins or formaldehyde-free acrylate-based binders meet all of the requirements placed thereon, such as printability, high interlaminar strength, good adhesion, and good coatability.
In order to adhesively bond the decorative films to wood materials, such as chipboard panels or MDF boards, urea-based glues or polyvinyl acetate (PVAC) glues are usually used.
High pressure laminates are laminates that are produced by pressing a number of impregnated papers layered on top of one another. The structure of this high pressure laminate consists generally of a transparent overlay, which produces a highest surface resistance, a decorative paper impregnated with resin, and one or more phenol-resinated kraft papers. By way of example, hardboards and wood chipboards as well as plywood are used as backings for this.
In the case of laminates produced by the short cycle method (low pressure laminates), the decorative paper impregnated with synthetic resin is pressed directly with a backing, for example a chipboard, under application of a low pressure.
The decorative paper used in the above-mentioned coating materials is used white or coloured with or without additional imprint.
With regard to the practical properties, the ‘decorative base papers’ serving as starting materials must meet certain requirements. These include a high opacity for an improved covering of the backing, uniform formation and grammage of the sheet for a uniform resin absorption, high light resistance, high purity and uniformity of colour for good reproducibility of the pattern to be printed on, high wet strength for frictionless impregnation, corresponding absorbability to attain the necessary degree of resin saturation, and dry strength, which is important during the rolling operations in the paper machine and when printing in the printing machine. Furthermore, the interlaminar strength (strength in the z-direction) is of particular importance, since it is a measure for how well the decorative base paper can be processed. The glued-on decorative paper/decorative film must not fray during machining steps such as sawing or drilling.
Decorative base papers generally consist of bright white sulphate pulps, predominantly of hardwood pulp, with a high proportion of pigments and fillers and wet strength agents, retention agents and fixing agents. Decorative base papers differ from conventional papers by the much higher filler content and the absence of an internal sizing or surface sizing with known sizing agents, such as alkyl ketene dimers, which is usual in the case of paper.
The opacity is one of the most important properties of the decorative base paper. This characterizes the covering power with respect to the backing.
The opacity is caused by light scattering at the pigment particles. For a high light scattering capability it is advantageous on the one hand to use pigment particles having a certain size and a narrow size distribution. Furthermore, it is also advantageous when the light-scattering pigment particles are distributed as uniformly as possible in the medium that is to be made opaque. Agglomerations of the pigment particles prevent the light scattering.
In particular when introducing pigments in paper production, an agglomeration of the pigment particles is usually observed, however, with the result that there are microscopic regions in the paper in which a very large amount of pigment particles are arranged tightly beside one another. Other regions in the sheet by contrast contain only few pigment particles, such that the light passes through such regions largely unhindered and not scattered. This non-uniform distribution results in a reduced opacity of the paper, which has to be compensated for by an increased use of the pigment. The pigment proportion, however, cannot be increased arbitrarily, since in this case the physical properties such as retention behaviour and pulp suspension, strengths, light fastness and resin absorption would likely be impaired.
Various proposals have been made to improve the uniformity of the distribution of pigment particles.
U.S. Pat. No. 4,608,401 describes a method for encapsulating titanium dioxide particles with a water-insoluble polymer in an aqueous suspension and the use of the obtained particles in paints. DE 199 61 964 A1 describes a method for producing an aqueous dispersion of composite particles, consisting of a fine-particle inorganic solid and a polymer. However, the described teachings cannot be used advantageously in decorative base papers, because on the one hand the attainable distances between the pigment particles are too small, and on the other hand the pore volume of the decorative base paper is reduced by the soft polymer latex constituents of these pigment preparations, which has a disadvantageous effect on the impregnability of the base paper.
GB 487 835 describes preparations of dyes and colour pigments with melamine formaldehyde condensation products as a constituent of paints.
DE 10 2013 100353 A1 describes a reactive composite formed from titanium dioxide, a binder, and at least one carrier. The carrier is preferably an inorganic material, to which the titanium dioxide particles are applied with a reactive binder in order to form the ‘reactive composite’. At least 80 mass % of the titanium dioxide particles preferably have a particle size of less than 5 μm, and at least 80 mass % of the carrier particles preferably have a particle size of less than 50 μm. The entire composite has a particle size of greater than 63 μm.